Hitherto, various tape-shaped products of thermal resins are known, but almost no proposals have been made regarding a tape-shaped product suitable for forming a belt including a planar tape portion provided with a multiplicity of holes for retaining another object thereat. As an example of such a belt including a planar tape portion provided with a multiplicity of holes for holding another object thereat, there is an endless belt retaining balls rollably thereon for a guide device for linear motion on a track. As disclosed in Japanese Laid-Open Patent Application (JP-A) 5-52217, such a belt includes ball-retaining portions intervening a plurality of balls arranged with prescribed intervals in a row, and a flexible connecting member for connection between respective ball-retaining portions.
For production of a belt for a ball chain (hereinafter sometimes referred as a ball chain belt), there are known a method of forming prescribed ball-retaining holes in an extruded tape, and a method of direct injection molding without such a tape product. An example of the former method is disclosed in JP-A 2001-74048 wherein an elongated flat tape product (i.e., a belt member) is preliminarily formed by extrusion and is cut in a prescribed length to form a row of holes for loosely retaining balls, and spacer portions are formed between adjacent retaining holes for retaining balls while using the balls as inserts. In a case of forming a tape product (a belt member) by extrusion of a synthetic resin and then forming ball-retaining holes for retaining balls rollably, it is difficult to obtain a strength sufficient for using this product as an endless belt subject to sliding movement. Further, adhesion between spacer portions formed by injection molding and the belt member is insufficient so as to cause dropping-off of the spacer portions. For this reason, for a purpose of ensuring a tensile strength and a flexural strength of the belt member, JP-A 2001-74048 also discloses a method of using two extruders for extruding a resin functioning as a reinforcing material and a resin coating the reinforcing material to form a tape portion through a common die, and an extrusion forming method of embedding reinforcing members, such as glass fiber, carbon fiber or ceramic fiber along parallel longitudinal edges of a flat band-shaped belt. However, the above-mentioned method of co-extruding two types of resins for forming a reinforcing member cannot provide a sufficient strength, and if a large ratio of stretching is applied thereto for providing an increased strength, thermal shrinkability becomes larger, so that this product is not suitable for such use as an endless belt for retaining balls rollably in a linear motion guide device. On the other hand, the fiber, such as glass fiber, carbon fiber or ceramic fiber, of a material different from belt-forming material cannot be sufficiently strongly bonded with the belt-forming material, whereby these materials are liable to form a gap therebetween due to various loads during use, and strength is rapidly lowered if this gap occurs, thus involving a problem regarding durability.
Further, in another method of producing a ball chain belt as disclosed in, e.g., JP-A 11-247856, ball frames having a diameter larger than that of balls used for the ball chain are aligned in projections at prescribed intervals in a metal mold for injection molding of synthetic resin, and a synthetic resin is injected into the metal mold to form a connecting belt with the ball frames aligned therein, followed by removal of the connecting belt from the metal mold and pushing-in of balls into the ball frames of the molded product so as to rollably retain the balls therein. According to this method, it is very difficult to develop a sufficient size accuracy, and even if a sufficient accuracy can be attained, metal mold production costs become very expensive. Further, the removal of this product from the mold is difficult, and a proportion of defectives is liable to be higher due to occurrence of fins around the holes.
In another method as disclosed in, e.g., JP-A 5-196037, a plurality of ball pieces disposed between balls and a connecting band, connecting the ball pieces and provided with ball holes for receiving the balls, are integrally formed by injection molding. During the injection method, resins injected out of respective gates are joined together at an intermediate point between the gates to form a weld, of which a strength is liable to be lowered.
As described above, there has not been provided a tape-shaped product suitable for forming a belt including a planar tape portion provided with a multiplicity of holes for retaining another object thereat. Further, production of the belt members according to the above-mentioned methods is complicated, and it is difficult to attain a desired strength being exhibited by the products.